1. Field of the Invention
The present invention relates to a vacuum booster assembly adapted for use in a vehicle brake system, and more particularly to a vacuum booster assembly the housing of which is constructed of front and rear shells of light weight sheet metal.
2. Discussion of the Background
To provide a light weight vacuum booster assembly, it is indispensable to lighten the housing for the booster assembly. On the other hand, sufficient rigidity and strength of the housing are required to resist a reaction force applied thereto in operation of the booster assembly. For such requirements, Japanese Patent Publication No. 58-101862 discloses a vacuum booster assembly which includes front and rear shells of synthetic resin each formed at its inner wall with a pair of sleeve-like projections and coupled to each other at their annular opening end portions, a power piston housed within the coupled shells and having a cylindrical portion axially slidably supported on the rear shell and a pressure receiving portion integral with the front end of the cylindrical portion, and a diaphragm member attached to the pressure receiving portion of the power piston and coupled over the cylindrical portion of the power piston at its inner peripheral rim, the diaphragm member being clamped at its outer peripheral rim between the annular opening end portions of the shells and clamped between the inner ends of the sleeve-like projections. In such an arrangement of the booster assembly, the front and rear shells are molded to form therein the sleeve-like projections, resulting in an increase in the manufacturing cost of the booster assembly. It is also difficult to ensure air-tightness at the outer peripheral rim of the diaphragm member.